1. Field of the Invention
The present invention relates to an antenna apparatus used mainly for radio apparatus for a portable device such as a smart keyless entry system.
2. Description of the Related Art
An antenna apparatus in the related art is configured as shown in FIG. 6 which illustrates a schematic configuration and a connecting state. Reference numeral 1 designates a loop antenna, reference numeral 3 designates a ground conductor, reference numeral 4 designates a dielectric substrate, reference numeral 5 designates a transmitter-receiver, reference numeral 6 designates a first matching circuit and reference numeral 7 designates a second matching circuit.
Operation of the antenna apparatus in the related art will now be described.
The ground conductor 3 is formed on the front face of the dielectric substrate 4. Impedance of the loop antenna 1 provided around the ground conductor 3 is adjusted by the first matching circuit 6 connected to one of power dispatching units of the loop antenna 1 and the second matching circuit 7 connected to other portion of the loop antenna 1 so as to be operated in a frequency band to be used. The transmitter-receiver 5 is connected to the loop antenna 1 whose impedance is adjusted, and signals are transmitted and received by the transmitter-receiver 5 (for example, JP-A-2003-8326)
An example of impedance characteristics and reflection characteristics of the power dispatching unit of the loop antenna 1 in the related art are shown in FIGS. 7A and 7B. In FIGS. 7A and 7B, reference numeral 10 designates an impedance characteristic, and reference numeral 11 is a circle showing Voltage Standing Wave Ratio (VSWR)=2. The impedance characteristic 10 of the power dispatching unit of the loop antenna 1 is such that the frequency band width is narrow when VSWR≦2, so that sufficient performance cannot be obtained.
The antenna apparatus of this type is required to be compact considering convenience of portability. However, the antenna apparatus in the related art has a structure which is difficult to secure a sufficient antenna length in a compact size. Since the length of the antenna is limited, there arises a problem that it is difficult to widen the frequency band to be used.